1. Field of the Invention
This invention relates to the use of non-aqueous fluidizing agents for the preparation of stable, pumpable bentonite clay slurries containing more than 15% to upwards of 50% solids by weight. In slurries made according to the present invention, there is no significant diminution of commercially valuable properties of the bentonite, particularly its application as a thickener in aqueous media.
2. Description of the Prior Art
Bentonite is a generic term of a number of aluminum silicate clay minerals which in native form appear as flakes or sheets, not unlike mica. Structurally, bentonites consist essentially of flat plates of aluminum-oxygen-silicon molecules laminated together like the pages of a book. In contact with water, bentonite swells as it xe2x80x9cunleafsxe2x80x9d. It is believed that a strong electrostatic force between the layers of the mineral draws in the polar liquid water and the sheet-like structure swells up to eight times its original size as water forces itself between the xe2x80x9cpagesxe2x80x9d.
Because of their very great water absorbency, bentonites result in a thickening of aqueous media to which they are added, affording a wide number of applications in the fields of paper-making, oil recovery, mining and tunnel formation, agriculture, and drilling muds to name only some major applications.
However, their very property of swelling in water upon which useful applications of bentonites are based, leads to serious problems in the handling, delivery and transportation of bentonite clays. Pumpable aqueous slurries of bentonites do not commonly exhibit a solids content of more than about 7%, above which settling or caking occurs which impairs or destroys the pumpability of the slurry. Not only is the maximum solids content of a pumpable aqueous bentonite slurry only about 7%, but stock aqueous slurries of bentonite must be kept agitated to prevent settling and caking which is very difficult to reverse.
By way of example, in current paper-making operations carried out at paper mill sites, bentonite clay is added to the head box after dilution from a concentration of about 7% solids to about 0.5% solids. This procedure requires significant expertise and care in order to maintain uniform swelling response properties on the paper line. Too, the operation naturally requires a large volume of make-up water. On-site make-up involves considerable space and costly equipment and servicing; a typical fabrication unit in a paper plant costs more than $150,000.
Another illustration of the limitations inherent in the use of aqueous bentonite slurries is afforded by the use of such slurries in field applications, such as tunnel stabilization. Bentonite must be pumped at the site at solids contents of only 1-5%. This has the desired effect of placing the bentonite into selected locations, but xe2x80x9cuses upxe2x80x9d a significant portion of the total water absorbency of the bentonite clay.
It has been a generally recognized desideratum to provide a suspension medium for bentonite slurries which itself is of high fluidity (therefore permitting enhanced bentonite solids content) but which does not interfere with the desired end-use subsequent swelling of bentonite by water. As noted above, as a carrier any aqueous medium causes swelling and uses up some of the absorbency.
A number of approaches are known which seek to resolve this problem through the addition of any of a wide variety of additives or processing stages. While some of these approaches have resulted in the preparation of higher solids slurriesxe2x80x94up to about 15% bentonite solidsxe2x80x94they generally increase costs considerably and usually harm subsequent use of the bentonite as a thickener. Additionally, the control of the swelling process is often not reproducible from batch to batch and problems of settling and caking are still present.
Attempts have been made to use non-aqueous fluids as carriers of the mineral, but these often brought their own problems such as flammability, toxicity, or interference with the commercial process in which bentonite serves as the thickener, e.g. paper-making. Some likely candidates as carriers proved useless because of almost instantaneous interaction with the bentonite to cause gelling or solidification of the mixture.
In our efforts to find a useful non-aqueous fluidizer which would allow the preparation of high solids, pumpable bentonite slurries, we concluded that, at the minimum, the following conditions should prevail:
(i) the fluidizing agent should be xe2x80x9cinertxe2x80x9d in the sense of minimal physical-chemical interactions between the fluid and the bentonite;
(ii) the fluidizing agent should not interfere with the normal and intended operation of the bentonite (retention, formation, pitch removal, viscosity increase, lubrication, etc);
(iii) the fluidizing agent should not be toxic or otherwise hazardous for use in the intended application of the mineral, e.g. in paper mills;
(iv) the fluidizing agent should allow slurries of significantly more than 15% bentonite solids to be prepared; and
(v) the fluidizing agent should be readily available at relatively low cost to permit its widespread use.
A particularly preferred fluid is highly saturated lubricating oil that has been subjected to repeated hydrogenation steps to produce an oil having a degree of saturation of about 95% or greater. An example of such an oil is M-65, CAS [chemical abstract services] No. 72623-86-0, which is a mixture of hydrotreated hydrocarbons containing 15 to 30 carbon atoms, CAS No. 72623-95-9, which is a mixture of hydrotreated hydrocarbons containing 20 to 50 carbon atoms, and CAS No. 72623-87-1, which is a mixture of hydrotreated hydrocarbons containing 20 to 50 carbon atoms.
The lubricating oil having CAS No. 72634-86-0 is obtained by treating light vacuum gas oil and heavy vacuum gas oil with hydrogen in the presence of a catalyst in a two stage process with dewaxing being carried out between the two stages. It consists essentially of hydrocarbons containing 15 to 30 carbon atoms. The lubricating oils having CAS No. 72623-85-0 and 72623-87-1 are obtained by treating light vacuum gas oil, heavy vacuum gas oil and deasphalted residual oil with hydrogen in the presence of a catalyst in a two stage process with dewaxing being carried out between the two stages. It consists essentially of hydrocarbons containing 20 to 50 carbon atoms.
With a view to achieving the objects and overcoming the problems aforementioned, the present invention provides according to one aspect thereof a high-solids, pumpable bentonite slurry which comprises a fluidizing agent consisting essentially of one or more hydrocarbon oils exhibiting a high degree of saturation, mixed with from about 15 to upwards of 50% by weight of bentonite clay.
According to another aspect of the invention, there is provided a method for preparing a high-solids pumpable and flowable bentonite slurry by stirring particulate bentonite clay into a non-aqueous fluidizing agent to a content of greater than about 50% bentonite solids, where the fluidizing agent consists essentially of highly saturated hydrocarbon oils of from 15 to 50 carbon atoms per molecule.